test/file-register.c: Add tests for skipping file
test/file-register.c: Add tests for skipping file

This commit add test_skip which tests that
IORING_REGISTER_FILES_UPDATE will succeed with fds set as
IORING_REGISTER_FILES_SKIP and that the underlying registered file is
unaffected.

Signed-off-by: Noah Goldstein <a9731957091dfe7eca8b7432bff5a416e09afb62@gmail.com>

axboe/liburing,axboe/liburing,axboe/liburing